I think you should check with the company you are intending to fly in
order to know if the flight stop at Shannon when going to Dublin, or
viceversa. It really depends on the company. I have seen it done
either way.

Shannon is smaller and more manageable than Dublin. Also, I find it
better if you intend to rent your car at the airport and drive away.
I donŽt think a lot of Limerick as a city. They are doing a hard work
trying to promote it, and in fact there is a nice pedestrians-only
area, an interesting art gallery, but not a lot of things to do or
see. It is still a very industrial city.

About car rentals, there are some irish companies that you should
think about ( Dan Dooley is one of them, IŽll see if I have an URL
somewhere ). They are usually cheaper than the multinational ones,
but still good. Thrifty has also good prizes.

If I was landing in Dublin, I wouldnŽt rent at the airport. I would
head into town (there is a bus service to Heuston Station and more
places in the city, and there are leaflets at the airport so you know
how much should your taxi cost, not a lot, BTW) either by bus or by
taxi.